1. Technical Field
The technical field relates to a wavelength converting apparatus for converting the wavelength of incident light and outputting the light using a nonlinear optical crystal.
2. Description of the Related Art
A wavelength converting apparatus that converts the wavelength of laser light using a nonlinear optical crystal is popular because the apparatus is capable of producing laser light having wavelengths that are difficult to produce using laser elements alone.
The efficiency of wavelength conversion of a nonlinear optical crystal generally increases in proportion to the square of the distance of laser light propagation. Therefore, the distance of laser light propagation inside a nonlinear optical crystal is preferably as long as possible. Meanwhile, making a longer nonlinear optical crystal results in increased cost and making large-sized apparatuses.
Then, a technique of making the distance of laser light propagation longer by deflecting laser light inside a nonlinear optical crystal is disclosed in Japanese Patent Application Laid-Open No. 2005-106954, for example.
FIG. 1 is a configuration diagram of the nonlinear optical crystal disclosed in the above patent document. According to the technique disclosed in the patent document, nonlinear optical crystal 10 has input face 11, a plurality of total reflection faces 12 and 13, and output face 14. Laser light 20 before wavelength conversion enters input face 11 at a ninety-degree incident angle. Total reflection faces 12 and 13 reverse the traveling direction of laser light 20 while shifting the position of the traveling direction of laser light 20 entering from input face 11. Output face 14 emits laser light subjected to wavelength conversion inside nonlinear optical crystal 10. This makes it possible to shuttle laser light 20 in the longitudinal direction of nonlinear optical crystal 10 and make the distance of laser light propagation longer in a state nonlinear optical crystal 10 is prevented from being longer.
However, according to the technique disclosed in the above patent document, there is a problem that the output of light subjected to wavelength conversion decreases.
The reason of the above problem is as follows. In nonlinear optical crystal 10, laser light 20 becomes a mixed light of a light whose wavelength is not converted (hereinafter “unconverted light”) and of a light whose wavelength is converted (hereinafter “converted light”). However, the refractive index of a nonlinear optical crystal depends on the wavelength, and therefore phase matching is lost between converted light and unconverted light when a mixed light propagates in directions other than the direction phase matching is maintained. With the technique disclosed in the above patent document, although phase matching between converted light and unconverted light is maintained before converted light and unconverted light reflect on total reflection face 12, phase matching is lost when converted light and unconverted light reflect on total reflection face 12 and propagate. Then, wavelength conversion is performed on the unconverted light on the return path in the state phase matching is lost. As a result, converted light subjected to wavelength conversion after reflection and converted light subjected to wavelength conversion before reflection interfere with each other, and therefore the output of converted light decreases. On the other hand, it is very difficult to make adjustment in order to prevent interference between converted light subjected to wavelength conversion after reflection and converted light subjected to wavelength conversion before reflection.